1. Field
Example embodiments relate to pixels of organic light emitting diode (OLED) display devices and the OLED display devices.
2. Description of the Related Art
In an organic light emitting diode (OLED) display device, a driving transistor included in each pixel generates a driving current based on a data voltage applied to the pixel, and an OLED included in the pixel emits light, based on the driving current, to display an image.
The driving transistor may have a hysteresis characteristic such that a characteristic curve of the driving transistor in a current frame is changed according to an operating state of the driving transistor in a previous frame. Thus, even when the driving transistor receives data voltages having the same voltage level, the driving transistor may generate driving currents having different current levels according to the operating state in the previous frame. Accordingly, in the OLED display device, even when the same data voltage is applied, in a current frame, to a pixel that represented a black color in a previous frame and a pixel that represented a white color in the previous frame, the pixels may have different luminances in the current frame. To prevent or reduce this luminance disuniformity caused by the hysteresis of the driving transistor in a typical OLED display device, respective driving transistors may be initialized to an on-bias state before pixels emit light. Thus, all of the driving transistors in the OLED display device may have substantially the same characteristic curve, thereby reducing the luminance disuniformity caused by the hysteresis. However, in this case, because the driving transistors are turned on in each and every frame, degradation of the driving transistors may be accelerated.
Further, respective driving transistors included in an OLED display device may have different threshold voltages due to a process variation, etc. Thus, to reduce threshold voltage variations among the respective driving transistors in the OLED display device, a threshold voltage compensation voltage corresponding to a threshold voltage of a driving transistor may be stored in each pixel, and a data voltage to which the threshold voltage compensation voltage is added may be applied to a gate of the driving transistor. However, in a typical OLED display device, when a threshold voltage compensation operation is initiated, an initial voltage level of the threshold voltage compensation voltage provided to the compensation capacitor may be changed due to charge sharing between the compensation capacitor and a parasitic capacitor of an OLED, and thus a final voltage level of the threshold voltage compensation voltage stored in the compensation capacitor may be different from a desired voltage level. This unwanted threshold voltage compensation voltage change may be worsened by a characteristic change of the parasitic capacitor of the OLED caused by degradation of the OLED. Further, this unwanted voltage change may further be worsened when an anode of the OLED is sufficiently initialized.